1. Field of the Invention
The present invention relates to a projection exposure apparatus used to manufacture micro-device such as semi-conductor elements, liquid crystal display elements, camera elements (CCDs) and thin-film magnetic heads, by a lithographic process, and more particularly, it relates to a method for measuring optical features of an exposure apparatus such as light intensity distribution of illumination light used for transferring a pattern of a mask (for example, a reticle) onto a substrate (for example, a wafer or a glass plate) and an imaging feature of a projection optical system.
2. Description of the Related Art
In the past, for example, when semi-conductor elements or liquid crystal display elements are manufactured by a lithographic process, there have been used projection exposure apparatuses in which a pattern on a mask or a reticle (referred to generically as xe2x80x9creticlexe2x80x9d hereinafter) is projected for exposure, through a projection exposure system, onto a sensitive substrate (such as a wafer) on which photo-resist is coated. In such projection exposure apparatuses, a high accurate imaging feature is required to form a fine or minute circuit pattern with high accuracy, and, high overlapping accuracy is required to put the pattern of the reticle to be currently exposure-treated on top of the patterns formed on the substrate during exposure for formation of previous layers, in order to successively form patterns of respective layers on the same area of the substrate in a laminated manner. Thus, an imaging feature of the projection exposure apparatus for focusing the reticle pattern onto the substrate must be adjusted with high accuracy, and, to this end, various methods for evaluating the imaging feature of the projection exposure apparatus prior to exposure treatment have been proposed.
As one of the methods for evaluating the imaging feature of the projection exposure apparatus, a method in is which a pattern of a test reticle on which a plurality of marks are formed is projected for exposure onto a substrate prior to actual production exposure and a developed test pattern image is observed has been utilized most popularly. However, in this method, since a preliminary exposure process and a preliminary developing process are required, longer time and much labor are required and a special device for measuring an image must be provided.
To eliminate such an inconvenience, there has been proposed a method in which a photo-electric sensor is provided on a stage on which a sensitive substrate is rested and a spatial image of a test pattern of a reticle formed through a projection optical system is directly observed on the basis of output from the sensor (for example, refer to Japanese Patent Laid-open No. 59-94032 (1984)) (U.S. Pat. No. 4,629,313). According to this method, the change in imaging feature caused by not only initial adjustment of the apparatus but also time-lapse change of the apparatus, or change in environmental conditions (such as atmospheric pressure and temperature), or absorption of illumination light of the imaging optical system, or change in conditions of the apparatus such as an illumination condition (solid angle and the like) to the reticle can easily be observed and the imaging feature can be corrected on the basis of the observed result. Recent projection exposure apparatuses include a mechanism for measuring the imaging feature of the projection optical system to carry out this method.
The following two methods for measuring the spatial image of the test pattern (referred to as xe2x80x9cmeasurement patternxe2x80x9d hereinafter, because this test pattern is used for measuring the imaging feature) formed on the reticle by using the photo-electric sensor provided on the substrate stage are generally used in combination with the above-mentioned imaging feature measuring mechanism of the projection exposure apparatus.
In one of the above methods (first method), the spatial image of the single measurement pattern is measured while illuminating only a limited area on the reticle near the measurement pattern, and, after the measurement is finished, the setting of a blind (field stop) in the illumination system is altered and the illumination area on the reticle is switched to a limited area on the reticle near the next measurement pattern, and then, the spatial image of the next measurement pattern is measured. Similar operations are repeated.
In the other method (second method), the spatial images of the measurement patterns are successively measured while illuminating substantially the entire surface of the reticle.
Recently, as the semi-conductor elements become high density and circuit patterns become more minutely, higher resolving power and higher overlapping accuracy have been required. To this end, a plurality of measurement patterns are disposed within the reticle and the imaging feature of the projection optical system is evaluated more strictly by using the measured results of the spatial images of the measurement patterns in order to adjust the imaging feature of the projection optical system with high accuracy.
However, when the above-mentioned first method is used, although deterioration of the imaging feature due to absorption of the illumination light in the projection optical system is prevented by not illuminating the undesired areas, as the number of the measurement patterns (i.e., the number of measuring points or positions) is increased, the switching between the illumination areas imposes a bad influence upon the through-put.
On the other hand, when the above-mentioned second method is used, although there is no reduction of the through-put, since the entire surface of the reticle is illuminated by the illumination light during the measurement of all of the measurement patterns, the illumination light is absorbed by the reticle and the projection optical system, thereby worsening the imaging feature.
Further, in the exposure apparatus, light intensity distribution of the illumination light on the exposure surface is also measured.
Now, a method for measuring the light intensity distribution of the illumination light on the exposure surface in the conventional exposure apparatus will be described hereinbelow.
An illumination area is adjusted so that the illumination area of the exposure surface of the exposure apparatus becomes a maximum effective area. In general, the maximum effective area is a rectangular area or a square area having a diagonal line corresponding to a diameter of a circle slightly smaller than the maximum illumination area of a projection lens, which maximum effective area is referred to as xe2x80x9centire illumination areaxe2x80x9d and is used as a measurement range for seeking the light intensity distribution, for example.
A photo-electric conversion means for providing the light intensity distribution measurement is rested on the wafer stage so that a light receiving surface of the photo-electric conversion means becomes flush with an exposure surface of a wafer, and, by scan-moving the stage on which the photo-electric conversion means is rested relative to the measurement range, light intensity within the measurement range is measured by the photo-electric conversion means. The light intensity distribution within the measurement range is determined on the basis of an output value from the photo-electric conversion means and positional information of the light receiving surface from a position measuring means for measuring the position of the stage. An example of the measurement of the light intensity distribution is disclosed in Japanese Patent Publication No. 1-39207 (1989) (U.S. Pat. No. Re. 32,795), for example. Since the greater the number of different positions within the measurement range at which the light intensity is measured, the more accurate light intensity distribution can be obtained within the measurement range, the light intensity distribution is measured at various positions within the measurement range by successively shifting (or scan-moving) the photo-electric conversion means within the measurement range.
In the above-mentioned conventional technique, while the light intensity distribution of the illumination light is being measured, the illumination light from a light source passes through the projection lens. Thus, a part of the illumination light is absorbed as heat by an optical member of the projection lens, with the result that a temperature of the optical member is increased. Accordingly, when the exposure process is to be started after the measurement of the light intensity distribution, the exposure process cannot be started until the optical member is cooled to a stable condition, with the result that productivity or operability of the exposure apparatus is worsened.
Recently, the entire illumination area of the exposure apparatus has been increased. Consequently, a time period required for measuring the light intensity distribution within the measurement range has also been increased, with the result that various influences due to the heat on the projection lens tend to be increased during the measurement of the light intensity distribution.
In order to minimize the influence due to the heat on the projection lens, for example, the measuring time period may be shortened. This can be achieved by reducing the number of positions to be measured. In this case, however, the density of data of the light intensity distribution obtained within the measurement range becomes rough, with the result that the light intensity distribution cannot be correctly measured within the measurement range.
As a method for reducing light incident on the projection lens, it is considered that an amount of light incident on the projection lens is reduced by controlling a power supply portion of an exposure light source or by providing a beam attenuating filter in an exposure light path. However, such a method is not effective as the measuring method, since the light intensity distribution within the measurement range at the exposure surface of the photosensitive substrate may be varied with the change in illumination condition.
The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide a measuring method in which, when an optical feature of an exposure apparatus is measured, an error caused due to absorption of illumination light can be reduced as much as possible, and an exposure apparatus in which such measurement is permitted.
Another object of the present invention is to provide a focusing feature (imaging feature) correcting method in which change in focusing feature due to absorption of illumination light of a mask and a projection optical system can be suppressed as much as possible, without affecting a bad influence upon through-put by the switching of illumination areas.
The other object of the present invention is to achieve measurement of light intensity distribution with high accuracy while eliminating an influence of illumination light incident on a projection lens.
To achieve the above object, the present invention provides a method for measuring an optical feature, which can be applied to an exposure apparatus comprising a projection optical system, an illumination system including a light source and adapted to send exposure illumination light to the projection optical system, and a substrate stage for supporting a sensitive substrate so that the sensitive substrate is exposed by the illumination light passed through the projection optical system, and wherein a shot area of the illumination light is defined on a plane corresponding to a top surface of the sensitive substrate.
In such a method, there is provided a photo-electric sensor having a light receiving portion shiftable within the shot area along the plane, and the illumination light passed through the projection optical system is detected at a plurality of positions within the shot area; meanwhile, the illumination light reaching the shot area excluding an area surrounding the light receiving portion is shielded.
The present invention further provides an exposure apparatus comprising a projection optical system, an illumination system including a light source and adapted to send exposure illumination light to the projection optical system, and a substrate stage for supporting a sensitive substrate so that the sensitive substrate is exposed by the illumination light passed through the projection optical system, and wherein a shot area of the illumination light are defined on a plane corresponding to a top surface of the sensitive substrate.
This apparatus includes a photo-electric sensor having a light receiving portion supported by a substrate stage for a shifting movement within the shot area along the plane, a blind disposed between the light source and the projection optical system and adapted to shield the illumination light reaching the shot area excluding an area surrounding the light receiving portion while the light receiving portion is being shifted along the plane, and a control system for obtaining an optical feature of the exposure apparatus on the basis of a signal from the photo-electric sensor.
The present invention further provides a focusing feature correcting method for correcting a focusing feature of a projection optical system for projecting an image of a pattern formed on a mask onto a substrate. This method includes a first step for preparing a mask having a plurality of mark patterns; a second step for illuminating an area including one mark pattern formed on the mask by illumination light to photo-electrically detect a projected image of the mark pattern passed through the projection optical system and for setting an illumination area so that a limited area including a position of the mark pattern being detected and a position of a next mark pattern is illuminated during detection of the projected image; a third step for calculating a focusing feature of the projection optical system on the basis of data of the projected image detected in the second step, after the second step was performed by at least one time; and a fourth step for correcting the focusing feature of the projection optical system on the basis of a calculated result obtained in the third step.
According to this method, in the second step, when the area including one mark pattern formed on the mask by the illumination light to photo-electrically detect the projected image of the mark pattern passed through the projection optical system, the illumination area is set so that the limited area including the position of the mark pattern being detected and the position of the next mark pattern is illuminated. After the second step was performed by one time or plural times, the third step calculating the focusing feature of the projection optical system on the basis of data of the respective projected images detected in the first step is performed, and, in the fourth step, the focusing feature of the projection optical system is corrected on the basis of the calculated result.
In this way, since the illumination area is set so that the limited area including the position of the mark pattern being detected and the position of the next mark pattern is illuminated during the photo-electric detection of the projected image of the mark pattern passed through the projection optical system, when the projected image of the next mark pattern is detected, because the illumination area is set to include the next mark pattern, the detection of the projected image of the next mark pattern can be started immediately, with the result that the switching of the illumination areas does not affect a bad influence upon the through-put. Further, since the illumination area is limited to the area including the single mark pattern or the limited area including the position of the mark pattern being detected and the position of the next mark pattern at the maximum, the change in the focusing feature due to the absorption of the illumination light of the mask and the projection optical system can be suppressed as much as possible.
The present invention further provides a projection exposure apparatus for illuminating a mask on which a pattern is formed by illumination light and for projecting and exposing an image of the pattern onto a sensitive substrate through a projection optical system. This apparatus comprises a mask stage for supporting a mask having a plurality of mark patterns; an illumination means having an illumination area defining means for defining an illumination area on the mask and adapted to cause illumination light to illuminate the illumination area on the mask defined by the illumination area defining means; a substrate stage on which a sensitive substrate is rested; a relative shift means for shifting the mask and the substrate stage relative to each other in a predetermined direction; a photo-electric detection means for photo-electrically detecting projected images of the mark patterns (formed on the mask) obtained by the projection optical system during the relative shifting movement; a control means for controlling the illumination area defining means so that a limited area including a position of the mark pattern being detected and a position of a next mark pattern is illuminated while one mark pattern on the mask is being detected by the photo-electric detection means, thereby changing the illumination area and including a calculation means for a focusing feature of the projection optical system on the basis of data of the projected images detected by the photo-electric detection means; and a focusing feature correcting means for correcting the focusing feature of the projection optical system on the basis of a calculated result from the calculation means.
According to this apparatus, in a condition that the predetermined illumination area on the mask (for example, an area near one mark pattern on the mask) defined by the illumination area defining means is illuminated by the illumination light, when the mask and the substrate stage are shifted relative to each other in the predetermined direction by the relative shift means, the projected image (spatial image) of the mark pattern on the mask projected on the substrate stage through the projection optical system is detected photo-electrically during the relative shifting movement. While the projected image of one mark pattern is being detected by the photo-electric detection means in this way, the control means controls the illumination area defining means so that the limited area including the position of the mark pattern being detected and the position of the next mark pattern is illuminated, thereby changing the illumination area. The detecting operation for detecting the projected image of the mark pattern and the simultaneous changing operation for changing the illumination area are successively performed regarding different mark patterns on the mask. When the detection of the projected images of the mark patterns is finished, the focusing feature of the projection optical system is calculated by the calculation means on the basis of the data of the projected images detected by the photo-electric detection means, and the focusing feature of the projection optical system is corrected by the focusing feature correction means on the basis of the calculated result from the calculation-means.
In this way, while the projected image of the mark pattern passed through the projection optical system is being detected photo-electrically, since the control means controls the illumination area defining means so that the limited area including the position of the mark pattern being detected and the position of the next mark pattern is illuminated, thereby changing the illumination area, when the projected image of the next mark pattern is detected, because the illumination area is set to include the next mark pattern to be detected, the detection of the projected image of the next mark pattern can be started immediately, with the result that the switching of the illumination areas does not affect a bad influence upon the through-put. Further, since the illumination area is limited to the area including the single mark pattern or the limited area including the position of the mark pattern being detected and the position of the next mark pattern at the maximum, the change in the focusing feature due to the absorption of the illumination light of the mask and the projection optical system can be suppressed as much as possible.
In this case, the illumination area defining means may be sufficient to set the illumination area having a predetermined shape (for example, rectangular shape) at any position on the mask. However, the illumination area defining means may include a rotating function for rotating the illumination area in a plane perpendicular to an optical axis of the illumination light. In this case, the degree of freedom of arrangement of the mark patterns on the mask can be improved.
Further, in this case, although the photo-electric detection means may directly detect the spatial image of the mark pattern photo-electrically, when the data of the projected images detected by the photo-electric detection means is integrated data of the spatial images of the mark patterns, the calculation means may differentiate the integrated data in a relative scanning direction. Also in this case, since the data of the spatial images of the mark patterns can be obtained, the calculation of the focusing feature on the basis of the data of the spatial images and the correction of the focusing feature of the projection optical system on the basis of the calculated result can be performed.
Normally, when the data of the projected images detected by the photo-electric detection means is the integrated data of the spatial images of the mark patterns, an opening portion corresponding to a dimension of the mark pattern on the mask is provided on the substrate stage and the mark pattern is photo-electrically detected through the opening portion. Thus, in this case, since a signal is not averaged unlike to a case where a signal is passed through a slit-shaped opening portion, a signal corresponding to the actual image can be obtained.
The focusing feature of the projection optical system may include various factors. And, the calculation means may calculate at least one of magnification, distortion, coma, spherical surface, focus, astigmatism and field curvature, and the focusing feature correction means may correct the focusing feature calculated by the calculation means. Such focusing feature correction means may be constituted by a relatively simple means such as a means for driving a part of lens elements forming the projection optical system in the optical axis direction or in the oblique direction or a means for adjusting pressure in closed space(s) between the lens elements.
The present invention further provides an exposure apparatus for projecting an image of a pattern on a mask onto a sensitive substrate through a projection optical system, which comprises a photo-electric conversion means for measuring light intensity distribution of illumination light for illuminating the sensitive substrate while shifting within a predetermined plane near an image plane of the projection optical system, a light shield means disposed substantially in conjugation with an exposure plane of the sensitive substrate and adapted to define an illumination area on the sensitive substrate, and a control means for controlling the light shield means in association with the shifting movement of the photo-electric conversion means so that the illumination area defined by the light shield means becomes smaller than a measurement range of the light intensity distribution when the light intensity distribution is measured.
The control means may control the light shield means in association with a shifting movement of a stage portion on which the photo-electric conversion means is provided.
In this case, a light amount of the illumination light passed through the projection optical system is regulated by the light shield means reducing a light amount of the illumination light incident on the projection optical system, and, by shifting the light shield means and the photo-electric conversion means in synchronous with each other, a light amount of the illumination light on the light receiving surface of the photo-electric conversion means at each measuring position within the measurement range permits the measurement of the light intensity distribution within the measurement range, in the same condition as that the light amount of the illumination passed through the projection optical system is not regulated by the light shield means.
The present invention further provides a measuring method for measuring light intensity distribution of illumination light for illuminating a sensitive substrate to project a pattern on a mask onto the sensitive substrate through a projection optical system, wherein a photo-electric conversion means is shifted within a measurement range at which the light intensity distribution is sought during measurement of the light intensity distribution so that an illumination area of the illumination light is made smaller than the measurement range and is changed in association with a position of the photo-electric conversion means during the measurement of the light intensity distribution.
In this case, a light amount of the illumination light passed through the projection optical system is regulated by a light shield means reducing a light amount of the illumination light incident on the projection optical system, and, by shifting the light shield means and the photo-electric conversion means in synchronous with each other, a light amount of the illumination light on the light receiving surface of the photo-electric conversion means at each measuring position within the measurement range permits the measurement of the light intensity distribution within the measurement range, in the same condition as that the light amount of the illumination passed through the projection optical system is not regulated by the light shield means.
By setting the illumination area by means of the light shield means, at least an area (and therearound) where the photo-electric conversion means is positioned is selected as the illumination area, and, by shifting the illumination area by driving the light shield means and at the same time by controlling to shift the light receiving surface of the photo-electric conversion means into the illumination area, the light amount of the illumination light on the light receiving surface of the photo-electric conversion means at each measuring position within the measurement range permits the measurement of the light intensity distribution within the measurement range, in the same condition as that the light amount of the illumination passed through the projection optical system is not regulated by the light shield means.